The present invention concerns a color picture tube, and more particularly it concerns the structure of a conductive film coated on the inner wall surface of the neck tube.
Generally, the vacuum envelope for the color picture tube is made of glass and in order to supply high voltage applied to the anode button to the anode of an electron gun structure housed within the vacuum envelope, a conductive film is formed on the inner wall surface of the vacuum envelope and a conductive spacer made of metal which is attached to the tip of the electron gun structure makes contact with the conductive film. The material for this conductive film is subject to very severe restrictions such as it should be resistant to high temperatures and high vacuum, be resistant to scratch, and have a good conductivity, a small gas discharge, and excellent adhesion. In addition, excellent workability at the time of coating and inexpensiveness are required for the material. Accordingly, there are limitations on the conductive materials which may be used for this purpose, and at present the mixture of a conductive material such as graphite and water glass is mainly used in the industry.
FIG. 1 is a partial sectional view of one example of a conventional color picture tube. In this figure, a vacuum envelope 1 made of glass comprises a panel portion 1a, a funnel portion 1b, and a neck tube 1c. A shadow mask 2 is positioned opposite to a fluorescent screen 3 coated on the inner surface of the panel portion 1a. A coating of a conductive film 4 covers the inner wall surface of the funnel portion 1b and the neck tube 1c of the vacuum envelope 1. An anode button 5 is provided for connecting external high voltage to the conductive film 4. A conductive spacer 7 has one end secured to the tip of an electron gun structure 6 and the other end in contact with the conductive film 4, so that high voltage applied to the anode button 5 is fed to the anode of the electron gun structure 6 via the conductive spacer 7. When the high voltage applied to the anode button 5 from an external high voltage generating circuit (not shown) is relayed to the anode of the electron gun structure 6 via the conductive film 4 and the conductive spacer 7, the current normally flowing into the electron gun 6 is extremely small. However, in the step of knocking during the manufacture of the picture tubes and the step of operational test in the television set, sparks generate at the electron gun 6 at which time a momentary maximum current of about 1000 A having a wave form as shown in FIG. 2 passes through the conductive film 4 and the conductive spacer 7. Since the resistance across the conductive film 4 and the conductive spacer 7 is several tens of ohms, energy consumed in the path across the conductive film 4 and the conductive spacer 7 at the time of the spark generation reaches about 10.sup.7 Joules momentarily, and the heat thus generated causes the contact point portion of the conductive film 4 to the conductive spacer 7 to spatter. When the color picture tube is operated in the television set, the potential difference between the electrodes constituting the main lens of the electron gun 6 exceeds about 20 kV. The distance between the electrodes is, however, only about 1 mm, so that even a very small amount of conductive dusts present in the tube and deposited on the electrodes generates spark which leads to spattering of the contact point portion of the conductive film 4 to the conductive spacer 7, thus greatly undermining the reliability of the picture tubes.
When the electron gun 6 is inserted into the neck tube 1c at the time the picture tube is being assembled, the conductive spacer 7 inevitably rubs a portion of the conductive film 4. On the other hand, the conventional conductive film 4 uses powdered graphite as the main conductive material and water glass as a binder. Such a conductive film 4 is soft and easily scratched off when rubbed on the surface. Not only is it extremely difficult to completely eliminate this scratching, but also it is unavoidable that the scratched dirts remain in the tube and cause spark discussed above.